Double-stack redox flow battery

ABSTRACT

Methods and systems are provided for a redox flow battery system. In one example, the redox flow battery system has a first redox flow battery and a second redox flow battery, stacked above and in contact with the first redox flow battery along a vertical axis of the redox flow battery system. The second redox flow battery may be coupled to the first redox flow battery via nesting detents. Furthermore, operation of the first redox flow battery and the second redox flow battery may be adjustable according to a power demand.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 17/209,084, entitled “DOUBLE-STACK REDOX FLOW BATTERY”, andfiled on Mar. 22, 2021. U.S. patent application Ser. No. 17/209,084claims priority to U.S. Provisional Application No. 63/025,222, entitled“DOUBLE-STACK REDOX FLOW BATTERY”, and filed on May 15, 2020. The entirecontents of the above-listed applications are hereby incorporated byreference for all purposes.

FIELD

The present description relates generally to methods and systems for aredox flow battery.

BACKGROUND AND SUMMARY

Redox flow batteries may be suitable for grid scale storage applicationsdue to their capability for scaling power and capacity independently, aswell as for charging and discharging over thousands of cycles withreduced performance losses in comparison to conventional batterytechnologies. An all-iron hybrid redox flow battery is particularlyattractive due to incorporation of low cost, earth-abundant materials.The iron redox flow battery (IFB) relies on iron, salt, and water forelectrolyte, thus comprising simple, earth abundant, and inexpensivematerials and eliminates incorporation of harsh chemicals therebyreducing an environmental footprint of the IFB.

However, the inventors herein have recognized when more than one IFB isneeded to meet an energy demand, a multi-IFB system may becomeincreasingly costly due to a multiplicity of a set of components, whereinclusion of the set of components is demanded in each IFB. For example,each IFB may include two pressure plates per IFB which may cause aweight of the multi-IFB system to become burdensome as a number of theIFBs increases. Furthermore, costly manufacturing of hardware specificto a configuration of the IFBs may further escalate overall costs andweight.

In one example, the issues described above may be addressed by a redoxflow battery system a first redox flow battery and a second redox flowbattery, stacked above and in contact with the first redox flow batteryalong a vertical axis of the redox flow battery system. The second redoxflow battery may be coupled to the first redox flow battery via nestingdetents. Furthermore, operation of the first redox flow battery and thesecond redox flow battery may be adjustable according to a power demand.In this way, an iron redox flow battery (IFB) system may be maintainedcompact and efficient without increasing costs and weight.

It should be understood that the summary above is provided to introducein simplified form a selection of concepts that are further described inthe detailed description. It is not meant to identify key or essentialfeatures of the claimed subject matter, the scope of which is defineduniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic of an example redox flow battery systemincluding a battery cell with electrodes and a membrane separator.

FIG. 2 shows a perspective view of a double-stack redox flow batterysystem.

FIG. 3 shows an exploded view of the double-stack redox flow batterysystem of FIG. 2 .

FIG. 4 shows a cut-away view of a redox flow battery adapted with acompression assembly.

FIG. 5 shows a front view of the double-stack redox flow battery system.

FIG. 6 shows a first side view of the double-stack redox flow batterysystem, indicating electrolyte flow paths.

FIG. 7 shows a second side view of the double-stack redox flow batterysystem, indicating a location of current collectors.

FIG. 8 shows a stacking configuration of more than one double-stackredox flow battery.

FIG. 9 shows a detailed view of a section of FIG. 8 .

FIGS. 2-9 are shown approximately to scale, however, other dimensionsmay be used as desired.

DETAILED DESCRIPTION

The following description relates to systems and methods formanufacturing a redox flow battery with reduced cost of storage. Theredox flow battery is shown in a schematic diagram in FIG. 1 with anintegrated multi-chamber tank having separate positive and negativeelectrolyte chambers. In some examples, the redox flow battery may be anall-iron flow battery (IFB) utilizing iron redox chemistry at both anegative and a positive electrode of the IFB. The electrolyte chambersmay be coupled to one or more battery cells, each cell including thenegative electrode and the positive electrode. To reduce a cost andweight of the IFB system when more than one IFB is incorporated, the IFBmay be configured with more than one cell stack, as illustrated in FIG.2 in a first embodiment of the IFB. The first embodiment of the IFB isdepicted in an exploded view in FIG. 3 . The one or more battery cellsmay be stacked along a common axis to form a cell stack which may besandwiched between pressure plates and compressed by compressionassemblies, as shown in FIG. 4 in a cut-away view of a second,embodiment of the IFB. The first embodiment of the IFB may be adouble-stack IFB having a sub-stack separator plate dividing a firstcell stack from a second cell stack. Electrolyte may flow in and out ofthe double-stack IFB via a plurality of inlets and outlets in at leastone of the pressure plates as shown in a front view of the double-stackIFB in FIG. 5 . Electrolyte flow paths through each cell stack of thedouble-stack IFB is depicted in FIG. 6 in a side view of the IFB andpositioning of current collectors in the double-stack IFB is shown inFIG. 7 . The double-stack IFB may be configured to be stackable, asshown in FIGS. 8 and 9 , thereby further reducing a footprint of anoverall IFB system.

FIGS. 2-9 show example configurations with relative positioning of thevarious components. If shown directly contacting each other, or directlycoupled, then such elements may be referred to as directly contacting ordirectly coupled, respectively, at least in one example. Similarly,elements shown contiguous or adjacent to one another may be contiguousor adjacent to each other, respectively, at least in one example. As anexample, components laying in face-sharing contact with each other maybe referred to as in face-sharing contact. As another example, elementspositioned apart from each other with only a space there-between and noother components may be referred to as such, in at least one example. Asyet another example, elements shown above/below one another, at oppositesides to one another, or to the left/right of one another may bereferred to as such, relative to one another. Further, as shown in thefigures, a topmost element or point of element may be referred to as a“top” of the component and a bottommost element or point of the elementmay be referred to as a “bottom” of the component, in at least oneexample. As used herein, top/bottom, upper/lower, above/below, may berelative to a vertical axis of the figures and used to describepositioning of elements of the figures relative to one another. As such,elements shown above other elements are positioned vertically above theother elements, in one example. As yet another example, shapes of theelements depicted within the figures may be referred to as having thoseshapes (e.g., such as being circular, straight, planar, curved, rounded,chamfered, angled, or the like). Further, elements shown intersectingone another may be referred to as intersecting elements or intersectingone another, in at least one example. Further still, an element shownwithin another element or shown outside of another element may bereferred as such, in one example.

Hybrid redox flow batteries are redox flow batteries that arecharacterized by the deposition of one or more of the electro-activematerials as a solid layer on an electrode. Hybrid redox flow batteriesmay, for instance, include a chemical that plates via an electrochemicalreaction as a solid on a substrate throughout the battery chargeprocess. During battery discharge, the plated species may ionize via anelectrochemical reaction, becoming soluble in the electrolyte. In hybridbattery systems, the charge capacity (e.g., a maximum amount of energystored) of the redox battery may be limited by the amount of metalplated during battery charge and may depend accordingly on theefficiency of the plating system as well as the available volume andsurface area available for plating.

As shown in FIG. 1 , in a redox flow battery system 10, a negativeelectrode 26 may be referred to as a plating electrode and a positiveelectrode 28 may be referred to as a redox electrode. A negativeelectrolyte within a plating side (e.g., a negative electrodecompartment 20) of a first battery cell 18 may be referred to as aplating electrolyte, and a positive electrolyte on a redox side (e.g. apositive electrode compartment 22) of the first battery cell 18 may bereferred to as a redox electrolyte.

Anode refers to the electrode where electro-active material loseselectrons and cathode refers to the electrode where electro-activematerial gains electrons. During battery charge, the positiveelectrolyte gains electrons at the negative electrode 26; therefore thenegative electrode 26 is the cathode of the electrochemical reaction.During discharge, the positive electrolyte loses electrons; thereforethe negative electrode 26 is the anode of the reaction. Alternatively,during discharge, the negative electrolyte and negative electrode may berespectively referred to as an anolyte and anode of the electrochemicalreaction, while the positive electrolyte and the positive electrode maybe respectively referred to as a catholyte and cathode of theelectrochemical reaction. During charge, the negative electrolyte andnegative electrode may be respectively referred to as the catholyte andcathode of the electrochemical reaction, while the positive electrolyteand the positive electrode may be respectively referred to as theanolyte and anode of the electrochemical reaction. For simplicity, theterms positive and negative are used herein to refer to the electrodes,electrolytes, and electrode compartments in redox battery flow systems.

One example of a hybrid redox flow battery is an all iron redox flowbattery (IFB), in which the electrolyte includes iron ions in the formof iron salts (e.g., FeCl₂, FeCl₃, and the like), wherein the negativeelectrode includes metal iron. For example, at the negative electrode26, ferrous ion, Fe²⁺, receives two electrons and plates as iron metalon to the negative electrode 26 during battery charge, and iron metal,Fe⁰, loses two electrons and re-dissolves as Fe²⁺ during batterydischarge. At the positive electrode, Fe²⁺ loses an electron to formferric ion, Fe³⁺, during charge, and during discharge Fe³⁺ gains anelectron to form Fe²⁺. The electrochemical reaction is summarized inequations (1) and (2), wherein the forward reactions (left to right)indicate electrochemical reactions during battery charge, while thereverse reactions (right to left) indicate electrochemical reactionsduring battery discharge:

Fe²⁺ + 2e− ↔Fe⁰ −0.44 V (Negative Electrode) (1) Fe²⁺↔ 2 Fe³⁺ + 2e−+0.77 V (Positive Electrode) (2)

As discussed above, the negative electrolyte used in the IFB may providea sufficient amount of Fe²⁺ so that, during charge, Fe²⁺ can accept twoelectrons from the negative electrode to form Fe⁰ and plate onto asubstrate. During discharge, the plated Fe⁰ may then lose two electrons,ionizing into Fe²⁺ and may be dissolved back into the electrolyte. Theequilibrium potential of the above reaction is −0.44 V and thus, thisreaction provides a negative terminal for the desired system. On thepositive side of the IFB, the electrolyte may provide Fe²⁺ during chargewhich loses an electron and oxidizes to Fe³⁺. During discharge, Fe³⁺provided by the electrolyte becomes Fe²⁺ by absorbing an electronprovided by the electrode. The equilibrium potential of this reaction is+0.77 V, creating a positive terminal for the desired system.

The IFB provides the ability to charge and recharge its electrolytes incontrast to other battery types utilizing non-regenerating electrolytes.Charge is achieved by applying a current across the electrodes viaterminals 40 and 42. The negative electrode 26 may be coupled viaterminal 40 to the negative side of a voltage source so that electronsmay be delivered to the negative electrolyte via the positive electrode(e.g., as Fe²⁺ is oxidized to Fe³⁺ in the positive electrolyte in thepositive electrode compartment 22). The electrons provided to thenegative electrode 26 (e.g., plating electrode) can reduce the Fe²⁺ inthe negative electrolyte to form Fe⁰ at the plating substrate, causingit to plate onto the negative electrode 26.

Discharge can be sustained while Fe⁰ remains available to the negativeelectrolyte for oxidation and while Fe³⁺ remains available in thepositive electrolyte for reduction. As an example, Fe³⁺ availability canbe maintained by increasing the concentration or the volume of thepositive electrolyte to the positive electrode compartment 22 side ofthe first battery cell 18 to provide additional Fe³⁺ ions via anexternal source, such as an external positive electrolyte tank 52. Morecommonly, availability of Fe⁰ during discharge may be an issue in IFBsystems, wherein the Fe⁰ available for discharge may be proportional tothe surface area and volume of the negative electrode substrate as wellas the plating efficiency. Charge capacity may be dependent on theavailability of Fe²⁺ in the negative electrode compartment 20. As anexample, Fe²⁺ availability can be maintained by providing additionalFe²⁺ ions via an external source, such as an external negativeelectrolyte chamber 50 to increase the concentration or the volume ofthe negative electrolyte to the negative electrode compartment 20 sideof the first battery cell 18.

In an IFB, the positive electrolyte comprises ferrous ion, ferric ion,ferric complexes, or any combination thereof, while the negativeelectrolyte comprises ferrous ion or ferrous complexes, depending on thestate of charge of the IFB system. As previously mentioned, utilizationof iron ions in both the negative electrolyte and the positiveelectrolyte allows for utilization of the same electrolytic species onboth sides of the battery cell, which can reduce electrolytecross-contamination and can increase the efficiency of the IFB system,resulting in less electrolyte replacement as compared to other redoxflow battery systems.

Efficiency losses in an IFB may result from electrolyte crossoverthrough a separator 24 (e.g., ion-exchange membrane barrier,micro-porous membrane, and the like). For example, ferric ions in thepositive electrolyte may be driven toward the negative electrolyte by aferric ion concentration gradient and an electrophoretic force acrossthe separator. Subsequently, ferric ions penetrating the membranebarrier and crossing over to the negative electrode compartment 20 mayresult in coulombic efficiency losses. Ferric ions crossing over fromthe low pH redox side (e.g., more acidic positive electrode compartment22) to high pH plating side (e.g., less acidic negative electrodecompartment 20) may result in precipitation of Fe(OH)₃. Precipitation ofFe(OH)₃ may degrade the separator 24 and cause permanent batteryperformance and efficiency losses. For example, Fe(OH)₃ precipitate maychemically foul the organic functional group of an ion-exchange membraneor physically clog the small micro-pores of an ion-exchange membrane. Ineither case, due to the Fe(OH)₃ precipitate, membrane ohmic resistancemay rise over time and battery performance may degrade. Precipitate maybe removed by washing the battery with acid, but the constantmaintenance and downtime may be disadvantageous for commercial batteryapplications. Furthermore, washing may be dependent on regularpreparation of electrolyte, contributing to additional processing costsand complexity. Alternatively, adding specific organic acids to thepositive electrolyte and the negative electrolyte in response toelectrolyte pH changes may mitigate precipitate formation during batterycharge and discharge cycling without driving up overall costs.Additionally, implementing a membrane barrier that inhibits ferric ioncross-over may also mitigate fouling.

Additional coulombic efficiency losses may be caused by reduction of H⁺(e.g., protons) and subsequent formation of H₂ (e.g., hydrogen gas), andthe reaction of protons in the negative electrode compartment 20 withelectrons supplied at the plated iron metal electrode to form hydrogengas.

The IFB electrolyte (e.g., FeCl₂, FeCl₃, FeSO₄, Fe₂(SO₄)₃, and the like)is readily available and can be produced at low costs. The IFBelectrolyte offers higher reclamation value because the same electrolytecan be used for the negative electrolyte and the positive electrolyte,consequently reducing cross contamination issues as compared to othersystems. Furthermore, owing to its electron configuration, iron maysolidify into a generally uniform solid structure during plating thereofon the negative electrode substrate. For zinc and other metals commonlyused in hybrid redox batteries, solid dendritic structures may formduring plating. The stable electrode morphology of the IFB system mayincrease the efficiency of the battery in comparison to other redox flowbatteries. Further still, iron redox flow batteries reduce the use oftoxic raw materials and can operate at a relatively neutral pH ascompared to other redox flow battery electrolytes. Accordingly, IFBsystems reduce environmental hazards as compared with all other currentadvanced redox flow battery systems in production.

Continuing with FIG. 1 , a schematic illustration of the redox flowbattery system 10 is shown. The redox flow battery system 10 may includethe first redox flow battery cell 18 fluidly connected to amulti-chambered electrolyte storage tank 110. The first redox flowbattery may generally include the negative electrode compartment 20,separator 24, and positive electrode compartment 22. The separator 24may comprise an electrically insulating ionic conducting barrier whichprevents bulk mixing of the positive electrolyte and the negativeelectrolyte while allowing conductance of specific ions therethrough.For example, the separator 24 may comprise an ion-exchange membraneand/or a microporous membrane.

The negative electrode compartment 20 may include the negative electrode26, and the negative electrolyte may be at least partially formed ofelectroactive materials. The positive electrode compartment 22 maycomprise the positive electrode 28, and the positive electrolyte maycomprise electroactive materials. In some examples, multiple redox flowbattery cells 18 may be combined in series or in parallel to generate ahigher voltage or current in a redox flow battery system. For example,in some examples, the redox flow battery system 10 may include two cellstacks, as shown in FIGS. 2-9 , where each cell stack is formed ofmultiple battery cells. As an example, the redox flow battery system 10is depicted in FIG. 1 with the first battery cell 18 as well as a secondbattery cell 19, similarly configured to the first battery cell 18. Assuch, all components and processes described herein for the firstbattery cell 18 may be similarly found in the second battery cell 19.

The first battery cell 18 may be included in a first cell stack and thesecond battery cell 19 may be included in a second cell stack. The firstand second cells may or may not be either fluidly coupled to one anotherbut are each fluidly coupled to the electrolyte storage tank 110 andrebalancing reactors 80, 82. For example, each of the first and secondbattery cells 18, 19 may be connected to negative and positiveelectrolyte pumps 30 and 32 via common passages that branch to each ofthe first and second battery cells 18 and 19, as shown in FIG. 1 .Similarly, the battery cells may each have passages that merge intocommon passages coupling the battery cells to the rebalancing reactors80, 82.

Further illustrated in FIG. 1 are the negative and positive electrolytepumps 30 and 32, both used to pump electrolyte solution through the flowbattery system 10. Electrolytes are stored in one or more tanks externalto the cell, and are pumped via the negative and positive electrolytepumps 30 and 32 through the negative electrode compartment 20 side andthe positive electrode compartment 22 side of the battery, respectively.

The redox flow battery system 10 may also include a first bipolar plate36 and a second bipolar plate 38, each positioned along a rear-facingside, e.g., opposite of a side facing the separator 24, of the negativeelectrode 26 and the positive electrode 28, respectively. The firstbipolar plate 36 may be in contact with the negative electrode 26 andthe second bipolar plate 38 may be in contact with the positiveelectrode 28. In other examples, however, the bipolar plates may bearranged proximate but spaced away from the electrodes within therespective electrode compartments. The IFB electrolytes may betransported to reaction sites at the negative and positive electrodes 26and 28 by the first and second bipolar plates 36 and 38, resulting fromconductive properties of a material of the bipolar plates 36, 38.Electrolyte flow may also be assisted by the negative and positiveelectrolyte pumps 30 and 32, facilitating forced convection through thefirst redox flow battery cell 18. Reacted electrochemical species mayalso be directed away from the reaction sites by the combination offorced convection and the presence of the first and second bipolarplates 36 and 38.

As illustrated in FIG. 1 , the first redox flow battery cell 18 mayfurther include negative battery terminal 40, and positive batteryterminal 42. When a charge current is applied to the battery terminals40 and 42, the positive electrolyte is oxidized (lose one or moreelectrons) at the positive electrode 28, and the negative electrolyte isreduced (gain one or more electrons) at the negative electrode 26.During battery discharge, reverse redox reactions occur on theelectrodes. In other words, the positive electrolyte is reduced (gainone or more electrons) at the positive electrode 28, and the negativeelectrolyte is oxidized (lose one or more electrons) at the negativeelectrode 26. The electrical potential difference across the battery ismaintained by the electrochemical redox reactions in the positiveelectrode compartment 22 and the negative electrode compartment 20, andmay induce a current through a current collector while the reactions aresustained. The amount of energy stored by a redox battery is limited bythe amount of electro-active material available in electrolytes fordischarge, depending on the total volume of electrolytes and thesolubility of the electro-active materials.

The flow battery system 10 may further include the integratedmulti-chambered electrolyte storage tank 110. The multi-chamberedstorage tank 110 may be divided by a bulkhead 98. The bulkhead 98 maycreate multiple chambers within the storage tank so that both thepositive and negative electrolyte may be included within a single tank.The negative electrolyte chamber 50 holds negative electrolytecomprising electroactive materials, and the positive electrolyte chamber52 holds positive electrolyte comprising electroactive materials. Thebulkhead 98 may be positioned within the multi-chambered storage tank110 to yield a desired volume ratio between the negative electrolytechamber 50 and the positive electrolyte chamber 52. In one example, thebulkhead 98 may be positioned to set the volume ratio of the negativeand positive electrolyte chambers according to the stoichiometric ratiobetween the negative and positive redox reactions. The figure furtherillustrates the fill height 112 of storage tank 110, which may indicatethe liquid level in each tank compartment. The figure also shows gashead space 90 located above the fill height 112 of negative electrolytechamber 50, and gas head space 92 located above the fill height 112 ofpositive electrolyte chamber 52. The gas head space 92 may be utilizedto store hydrogen gas generated through operation of the redox flowbattery (e.g., due to proton reduction and corrosion side reactions) andconveyed to the multi-chambered storage tank 110 with returningelectrolyte from the first redox flow battery cell 18. The hydrogen gasmay be separated spontaneously at the gas-liquid interface (e.g., fillheight 112) within the multi-chambered storage tank 110, therebyprecluding having additional gas-liquid separators as part of the redoxflow battery system. Once separated from the electrolyte, the hydrogengas may fill the gas head spaces 90 and 92. As such, the stored hydrogengas can aid in purging other gases from the multi-chamber storage tank110, thereby acting as an inert gas blanket for reducing oxidation ofelectrolyte species, which can help to reduce redox flow batterycapacity losses. In this way, utilizing the integrated multi-chamberedstorage tank 110 may forego having separate negative and positiveelectrolyte storage tanks, hydrogen storage tanks, and gas-liquidseparators common to conventional redox flow battery systems, therebysimplifying the system design, reducing the physical footprint of thesystem, and reducing system costs.

FIG. 1 also shows the spill over-hole 96, which creates an opening inthe bulkhead 98 between gas head spaces 90 and 92, and provides a meansof equalizing gas pressure between the two chambers. The spill over hole96 may be positioned a threshold height above the fill height 112. Thespill over hole further enables a capability to self-balance theelectrolytes in each of the positive and negative electrolyte chambersin the event of a battery crossover. In the case of an all iron redoxflow battery system, the same electrolyte (Fe′) is used in both negativeand positive electrode compartments 20 and 22, so spilling over ofelectrolyte between the negative and positive electrolyte chambers 50and 52 may reduce overall system efficiency, but the overall electrolytecomposition, battery module performance, and battery module capacity aremaintained. Flange fittings may be utilized for all piping connectionsfor inlets and outlets to and from the multi-chambered storage tank 110to maintain a continuously pressurized state without leaks. Themulti-chambered storage tank 110 can include at least one outlet fromeach of the negative and positive electrolyte chambers, and at least oneinlet to each of the negative and positive electrolyte chambers.Furthermore, one or more outlet connections may be provided from the gashead spaces 90 and 92 for directing hydrogen gas to rebalancing reactors80 and 82.

Although not shown in FIG. 1 , integrated multi-chambered electrolytestorage tank 110 may further include one or more heaters thermallycoupled to each of the negative electrolyte chamber 50 and the positiveelectrolyte chamber 52. In alternate examples, only one of the negativeand positive electrolyte chambers may include one or more heaters. Inthe case where only the positive electrolyte chamber 52 includes one ormore heaters, the negative electrolyte may be heated by transferringheat generated at the battery cells of the power module to the negativeelectrolyte. In this way, the battery cells of the power module may heatand facilitate temperature regulation of the negative electrolyte. Theone or more heaters may be actuated by the controller 88 to regulate atemperature of the negative electrolyte chamber 50 and the positiveelectrolyte chamber 52 independently or together. For example, inresponse to an electrolyte temperature decreasing below a thresholdtemperature, the controller 88 may increase a power supplied to one ormore heaters so that a heat flux to the electrolyte is increased. Theelectrolyte temperature may be indicated by one or more temperaturesensors mounted at the multi-chambered electrolyte storage tank 110,including sensors 60 and 62. As examples, the one or more heaters mayinclude coil type heaters or other immersion heaters immersed in theelectrolyte fluid, or surface mantle type heaters that transfer heatconductively through the walls of the negative and positive electrolytechambers to heat the fluid therein. Other known types of tank heatersmay be employed without departing from the scope of the presentdisclosure. Furthermore, controller 88 may deactivate one or moreheaters in the negative and positive electrolyte chambers 50, 52 inresponse to a liquid level decreasing below a solids fill thresholdlevel. Said in another way, controller 88 may activate the one or moreheaters in the negative and positive electrolyte chambers 50, 52 only inresponse to a liquid level increasing above the solids fill thresholdlevel. In this way, activating the one or more heaters withoutsufficient liquid in the positive and/or negative electrolyte chamberscan be averted, thereby reducing a risk of overheating or burning outthe heaters.

Further still, one or more inlet connections may be provided to each ofthe negative and positive electrolyte chambers 50, 52 from a fieldhydration system (not shown). In this way, the field hydration systemcan facilitate commissioning of the redox flow battery system, includinginstalling, filling, and hydrating the system, at an end-use location.Furthermore, prior to its commissioning at the end-use location, theredox flow battery system may be dry-assembled at a batterymanufacturing facility different from end-use location without fillingand hydrating the system, before delivering the system to the end-uselocation. In one example, the end-use location may correspond to thelocation where the redox flow battery system 10 is to be installed andutilized for on-site energy storage. Said in another way, it isanticipated that, once installed and hydrated at the end-use location, aposition of the redox flow battery system 10 becomes fixed, and theredox flow battery system 10 is no longer deemed a portable, dry system.Thus, from the perspective of a redox flow battery system end-user, thedry portable redox flow battery system 10 may be delivered on-site,after which the redox flow battery system 10 is installed, hydrated andcommissioned. Prior to hydration the redox flow battery system 10 may bereferred to as a dry, portable system, the redox flow battery system 10being free of or without water and wet electrolyte. Once hydrated, theredox flow battery system 10 may be referred to as a wet non-portablesystem, the redox flow battery system 10 including wet electrolyte.

Further illustrated in FIG. 1 , electrolyte solutions typically storedin the multi-chambered storage tank 110 are pumped via negative andpositive electrolyte pumps 30 and 32 throughout the flow battery system10. Electrolyte stored in negative electrolyte chamber 50 is pumped vianegative electrolyte pump 30 through the negative electrode compartment20 side, and electrolyte stored in positive electrolyte chamber 52 ispumped via positive electrolyte pump 32 through the positive electrodecompartment 22 side of the battery.

Two electrolyte rebalancing reactors 80 and 82, may be connected in-lineor in parallel with the recirculating flow paths of the electrolyte atthe negative and positive sides of the first battery cell 18,respectively, in the redox flow battery system 10. One or morerebalancing reactors may be connected in-line with the recirculatingflow paths of the electrolyte at the negative and positive sides of thebattery, and other rebalancing reactors may be connected in parallel,for redundancy (e.g., a rebalancing reactor may be serviced withoutdisrupting battery and rebalancing operations) and for increasedrebalancing capacity. In one example, the electrolyte rebalancingreactors 80 and 82 may be placed in the return flow path from thepositive and negative electrode compartments 20 and 22 to the positiveand negative electrolyte chambers 50 and 52, respectively. Electrolyterebalancing reactors 80 and 82 may serve to rebalance electrolyte chargeimbalances in the redox flow battery system occurring due to sidereactions, ion crossover, and the like, as described herein. In oneexample, electrolyte rebalancing reactors 80 and 82 may include tricklebed reactors, where the hydrogen gas and electrolyte are contacted atcatalyst surfaces in a packed bed for carrying out the electrolyterebalancing reaction. In other examples the rebalancing reactors 80 and82 may include flow-through type reactors that are capable of contactingthe hydrogen gas and the electrolyte liquid and carrying out therebalancing reactions in the absence a packed catalyst bed.

During operation of the redox flow battery system 10, sensors and probesmay monitor and control chemical properties of the electrolyte such aselectrolyte pH, concentration, state of charge, and the like. Forexample, as illustrated in FIG. 1 , sensors 62 and 60 maybe bepositioned to monitor positive electrolyte and negative electrolyteconditions at the positive electrolyte chamber 52 and the negativeelectrolyte chamber 50, respectively. In another example, sensors 62 and60 may each include one or more electrolyte level sensors to indicate alevel of electrolyte in the positive electrolyte chamber 52 and thenegative electrolyte chamber 50, respectively. As another example,sensors 72 and 70, also illustrated in FIG. 1 , may monitor positiveelectrolyte and negative electrolyte conditions at the positiveelectrode compartment 22 and the negative electrode compartment 20,respectively. The sensors 72, 70 may be pH probes, optical probes,pressure sensors, voltage sensors, etc. Sensors may be positioned atother locations throughout the redox flow battery system 10 to monitorelectrolyte chemical properties and other properties.

For example, a sensor may be positioned in an external acid tank (notshown) to monitor acid volume or pH of the external acid tank, whereinacid from the external acid tank is supplied via an external pump (notshown) to the redox flow battery system 10 in order to reduceprecipitate formation in the electrolytes. Additional external tanks andsensors may be installed for supplying other additives to the redox flowbattery system 10. For example, various sensors including, temperature,conductivity, and level sensors of a field hydration system may transmitsignals to the controller 88. Furthermore, controller 88 may sendsignals to actuators such as valves and pumps of the field hydrationsystem during hydration of the redox flow battery system 10. Sensorinformation may be transmitted to a controller 88 which may in turnactuate pumps 30 and 32 to control electrolyte flow through the firstbattery cell 18, or to perform other control functions, as an example.In this manner, the controller 88 may be responsive to, one or acombination of sensors and probes.

Redox flow battery system 10 may further comprise a source of hydrogengas. In one example the source of hydrogen gas may comprise a separatededicated hydrogen gas storage tank. In the example of FIG. 1 , hydrogengas may be stored in and supplied from the integrated multi-chamberedelectrolyte storage tank 110. Integrated multi-chambered electrolytestorage tank 110 may supply additional hydrogen gas to the positiveelectrolyte chamber 52 and the negative electrolyte chamber 50.Integrated multi-chambered electrolyte storage tank 110 may alternatelysupply additional hydrogen gas to the inlet of electrolyte rebalancingreactors 80 and 82. As an example, a mass flow meter or other flowcontrolling device (which may be controlled by controller 88) mayregulate the flow of the hydrogen gas from integrated multi-chamberedelectrolyte storage tank 110. The integrated multi-chambered electrolytestorage tank 110 may supplement the hydrogen gas generated in redox flowbattery system 10. For example, when gas leaks are detected in redoxflow battery system 10 or when the reduction reaction rate is too low atlow hydrogen partial pressure, hydrogen gas may be supplied from theintegrated multi-chambered electrolyte storage tank 110 in order torebalance the state of charge of the electro-active species in thepositive electrolyte and negative electrolyte. As an example, controller88 may supply hydrogen gas from integrated multi-chambered electrolytestorage tank 110 in response to a measured change in pH or in responseto a measured change in state of charge of an electrolyte or anelectro-active species.

For example, an increase in pH of the negative electrolyte chamber 50,or the negative electrode compartment 20, may indicate that hydrogen isleaking from the redox flow battery system 10 and/or that the reactionrate is too slow with the available hydrogen partial pressure, andcontroller 88, in response to the pH increase, may increase a supply ofhydrogen gas from integrated multi-chambered electrolyte storage tank110 to the redox flow battery system 10. As a further example,controller 88 may supply hydrogen gas from integrated multi-chamberedelectrolyte storage tank 110 in response to a pH change, wherein the pHincreases beyond a first threshold pH or decreases beyond a secondthreshold pH. In the case of an IFB, controller 88 may supply additionalhydrogen to increase the rate of reduction of ferric ions and the rateof production of protons, thereby reducing the pH of the positiveelectrolyte. Furthermore, the negative electrolyte pH may be lowered byhydrogen reduction of ferric ions crossing over from the positiveelectrolyte to the negative electrolyte or by protons, generated at thepositive side, crossing over to the negative electrolyte due to a protonconcentration gradient and electrophoretic forces. In this manner, thepH of the negative electrolyte may be maintained within a stable region,while reducing the risk of precipitation of ferric ions (crossing overfrom the positive electrode compartment) as Fe(OH)₃.

Other control schemes for controlling the supply rate of hydrogen gasfrom integrated multi-chambered electrolyte storage tank 110 responsiveto a change in an electrolyte pH or to a change in an electrolyte stateof charge, detected by other sensors such as an oxygen-reductionpotential (ORP) meter or an optical sensor, may be implemented. Furtherstill, the change in pH or state of charge triggering the action ofcontroller 88 may be based on a rate of change or a change measured overa time period. The time period for the rate of change may bepredetermined or adjusted based on the time constants for the redox flowbattery system 10. For example, the time period may be reduced if therecirculation rate is high, and local changes in concentration (e.g.,due to side reactions or gas leaks) may quickly be measured since thetime constants may be small.

As described above, an IFB system, e.g., the flow battery system 10 ofFIG. 1 , may include a plurality of battery cells, such as the firstbattery cell 18 and the second battery cell 19 of FIG. 1 , stacked alonga common axis and forming one or more cell stacks. An IFB of the IFBsystem may further include a variety of hardware providing structuralsupport to the IFB and enabling an assembly of the hardware and the cellstack into a transportable unit. In some examples, multiple IFBs may beused to accommodate an energy demand, the multiple IFBs combined in astack and linked to a common energy consuming device. However, as anumber of IFBs in the IFB system increases, a weight and cost incurredby components, such as two pressure plates and four tube leaf springsper IFB, may become undesirably high. The cost and weight burden may beat least partially addressed by combining two IFBs into a single,double-stack IFB, the double-stack IFB incorporating a reduced number ofhardware components relative to two individual, single-stack IFB s.

An example of a double-stack IFB 202 is illustrated in FIG. 2 from aperspective view 200 and from an exploded view 300 in FIG. 3 . A set ofreferences axes 201 are provided for comparison between views. In oneexample, the z-axis is parallel with a longitudinal axis of the IFB 202.The IFB 202 has a first pressure plate 204 at a first end 206 of the IFB202 and a second pressure plate 208 at a second end 210 of the IFB 202.

A first cell stack 212 and a second cell stack 214 may be disposedbetween the first and second pressure plates 204, 208. Each cell of thefirst cell stack 212 and each cell of the second cell stack 214 may besimilar to the first and second battery cells 18 and 19 of FIG. 1 . Inone example, the first cell stack 212 may be formed of 50 cells and thesecond cell stack 214 may also be formed of 50 cells. However, otherexamples may include cell stacks with varying quantities of cells andthe cell stacks may or may not have the same number of cells. In yetother examples, the cell stacks may have more or less than 50 cells.Furthermore, other examples of an IFB may include different numbers ofcells stacks incorporated into the IFB. For example, the IFB may haveone cell stack or three cell stacks.

The first cell stack 212 may have a plurality of cells aligned along thez-axis and the second cell stack 214 may similarly include a pluralityof cells aligned along the z-axis. The first cell stack 212 is alsoaligned with the second cell stack 214 along the z-axis and the cellstacks may be separated by a sub-stack separator plate 216 that provideselectrical isolation between the cell stacks. In other words, the firstcell stack 212 may be sandwiched between the first pressure plate 204and a first face of the sub-stack separator plate 216 and the secondcell stack 214 may be sandwiched between a second face of the sub-stackseparator plate 216 and the second pressure plate 208. Details of thesub-stack separator plate 216 will be discussed further below.

Turning now to FIG. 3 , elements of the double-stack IFB 202 are nowdescribed along a direction from the first end 206 towards the secondend 210. Reference to an interior 302 of the IFB 202 indicates a regionbetween the first pressure plate 204 and the second pressure plate 208.The first cell stack 212 includes a first end plate 304 positionedinside of the first pressure plate 204 and in face-sharing contact withan interior surface of the first pressure plate 204. In other words, asurface of the first pressure plate 204 facing the interior 302 of theIFB 202 is in contact with a surface of the first end plate 304 facingaway from the interior 302 of the IFB 202. A first current collector306, configured to flow electrical current, may be arranged between thefirst end plate 304 and the first pressure plate 204.

A first bipolar plate assembly 308 is arranged between the first endplate 304 and a second end plate 310 of the first cell stack 212. Thebipolar plate assembly 308 includes a plurality of frame plates 312stacked along the z-axis, the plurality of frame plates 312 providingstructural support to the first and second cell stacks 212, 214. Eachframe plate of the plurality of frame plates 312 is similarly configuredto frame a cell of the first cell stack 212 and the second cell stack214. Each cell includes at least one bipolar plate 314 inserted into atleast one opening of each frame plate. Furthermore, the bipolar plate314 is positioned between a negative electrode and a positive electrode(not shown in FIG. 3 ) of each cell, the electrodes arranged alongopposite faces of the bipolar plate. In addition, the negative electrodeis positioned between the bipolar plate 314 and a membrane separator(e.g., the separator 24 of FIG. 1 ). In this way, each frame plate has astack of components including the membrane separator, the negativeelectrode, the bipolar plate 314, and the positive electrode, and thestack of components is repeated with each successive frame plate in thefirst and second cell stacks 212 and 214.

The second end plate 310 of the first cell stack 212 may be inface-sharing contact with a first surface of the sub-stack separatorplate 216, the first surface facing the first end 206 of the IFB 202. Inother words, a surface of the second endplate 310 facing the second end210 of the IFB 202 is in contact with the first surface of the sub-stackseparator plate 216. A second current collector 316 may be arrangedbetween the second end plate 310 and the sub-stack separator plate 216.

The second cell stack 214 may similarly extend along the z-axis betweenend plates. For example, as depicted in FIG. 7 , the second cell stack214 may have a third end plate 330 at the second end 310 of the IFB 302and a fourth end plate 332 adjacent to the sub-stack separator plate216. The fourth end plate 332 may be arranged on an opposite side of thesub-stack separator plate 216 from the second end plate 310. As such, asurface of the fourth end plate 332 facing the first end 206 of the IFB202 is in face-sharing contact with a second side of the sub-stackseparator plate 216, the second side facing the second end 210 of theIFB 202.

The second cell stack 214 includes a second bipolar assembly similar tothe first bipolar assembly 308. Furthermore, the second cell stack 214may be equivalently configured to the first cell stack 212 when thefirst cell stack 212 is rotated about the y-axis by 180 degrees. A thirdcurrent collector may be arranged between the sub-stack separator plate216 and the end plate of second cell stack 214 proximate to thesub-stack separator plate 216. A surface of the end plate of the secondcell stack 214 proximate to the second pressure plate 208 may be inface-sharing contact with a surface of the second pressure plate 208that is facing the first end 206 of the IFB 202.

The end plates of the cell stacks may provide solid end walls to sealfluids, such as electrolyte, inside the cell stacks. The first andsecond cell stacks 212, 214 may be secured between and to the first andsecond pressure plates 204, 208, by a variety of hardware, including,for example, tie rods 318, nuts 320, and tube leaf springs 322. The tubeleaf springs 322 may include rectangular tubes extending across outersurfaces of the first and second pressure plates 204, 208, along they-axis. Details of the tube leaf springs 322 are described next withreference to FIG. 4 .

Compression of an IFB by the hardware described above is depicted in acut-away view 400 of an alternate embodiment of an IFB 402 in FIG. 4 .The IFB 402 is cut along the y-z plane. While FIG. 4 shows the IFB 402with a single stack for brevity, hardware enabling compression of theIFB 402 may be similarly applied to a double-stack IFB. As describedabove, the hardware includes compression assemblies 404 which providescompression mechanisms for the IFB 402. Each of the compressionassemblies 404 is formed of a first leaf spring 406 extending along thex-axis across a first pressure plate 408, a second leaf spring 410extending along the x-axis across a second pressure plate 412, and tierods 414 extending along the z-axis and engaging with both the firstleaf spring 406 and the second leaf spring 410.

For example, the tie rods 414 may be inserted through openings in eachof the first and second leaf springs 406, 412 and tightening using nuts416 which may engage with the tie rods 414 via threading. As the nuts416 are tightened around the tie rods 414, a compressive force isexerted on the first and second pressure plates 408, 412 by the firstand second leaf springs 406, 412, as indicated by arrows 418. Each ofthe first and second leaf springs 406, 412 have fulcrums, indicated bydashed circles 420, formed in the first and second pressure plates 408,412, which spread a load exerted on the pressure plates by the leafsprings uniformly across the pressure plates. In this way, thecompression assemblies 404 reduce deflection of the IFB cell stack(s)when the IFB 402 is compressed.

A configuration of a double-stack IFB, such as the double-stack IFB 202shown in FIGS. 2-3 , with each cell of each cell stack supported by aplurality of frame plates, the cell stacks braced by a first pressureplate and a second pressure plate, the pressure plates coupled to oneanother by fasteners and compressing the IFB via compression assembliesas described above, allows the double-stack IFB to be self-supporting.In other words, the double-stack IFB does not rely on an outer housingto maintain components of the IFB, which may otherwise add to a weightand cost of the double-stack IFB. The double-stack IFB is thereby arobust, readily transportable unit configured to be compressed withreduced deflection.

Operation of the double-stack IFB may vary depending on how electrolyteis adapted to flow through the double-stack IFB. An exampleconfiguration of electrolyte flow through the double-stack IFB isdepicted in FIG. 5 . A front view 500 of the double-stack IFB 202 ofFIG. 2 is shown in FIG. 5 , showing the first pressure plate 204. Inanother example, the view shown in FIG. 5 may be a rear view of thedouble-stack IFB 202 depicting the second pressure plate 208. In otherwords, the first pressure plate 204 and the second pressure plate 208may be similarly configured and the following description of aspects ofthe first pressure plate 204 as well as electrolyte flow through thefirst cell stack 212 of the double-stack IFB 202 may be applied to thesecond pressure plate 208 and the second cell stack 214.

The first pressure plate 204, arranged at the first end 206 of the IFB202, as shown in FIG. 2 , may have a plurality of ports 502 extendingthrough a thickness of the first pressure plate 204 where the thicknessis defined along the z-axis. In one example, the first end 206 of theIFB 202 may be a positive end of the first cell stack 212 of the IFB202. A first port 504 of the plurality of ports 502 may be a negativeinlet, flowing negative electrolyte into the IFB 202. A second port 506of the plurality of ports 502 may be a positive inlet, flowing positiveelectrolyte into the IFB 202. A third port 508 of the plurality of ports502 may be a negative outlet port, flowing negative electrolyte out ofthe IFB 202, and a fourth port 510 of the plurality of ports 502 may bea positive outlet port, flowing positive electrolyte out of the IFB 202.As such, all electrolyte entering the first cell 212 of the IFB 202 mayenter through the first pressure plate 204 and all electrolyte leavingthe IFB 202 may also exit through the first pressure plate 204. In oneexample, electrolyte enters and exits the first cell 212 only throughthe first pressure plate 204.

Both the first port 504 and the second port 506 may be positioned at abottom end 512, with respect to the y-axis, of the IFB 202, alignedalong the x-axis. The third port 508 and the fourth port 510 may bepositioned at a top end 514, with respect to the y-axis, of the IFB 202,also aligned along the x-axis. Thus, negative electrolyte follows anegative flow path into the first cell stack 212 of the IFB 202 at thefirst port 504 along a first direction from the first pressure plate 204to the sub-stack separator plate 216, as shown in FIG. 6 in a first sideview 600 of the IFB 202 and indicated by arrows 602. The negative flowpath may turn to the left (with respect to the front view 500 shown inFIG. 5 ) and upwards (along the y-axis) as indicated by arrows 604 atmore than one location along the z-axis between the first pressure plate204 and the sub-stack separator plate 216. The flow path may makeanother perpendicular turn to flow along a second direction from thesub-stack separator plate 216 to the first pressure plate 204, asindicated by arrows 606, to emerge from the third port 508.

Similarly, positive electrolyte follows a positive flow path into thefirst cell stack 212 of the IFB 202 at the second port 506 along thefirst direction, indicated by arrows 602 in FIG. 6 , the positive flowpath turning to the right (with respect to the front view 500 shown inFIG. 5 ) and upwards (along the y-axis) at locations along the z-axisbetween the first pressure plate 204 and the sub-stack separator plate216. The flow path turns again, perpendicularly, to flow along thesecond direction, as indicated by arrows 606 in FIG. 6 , from thesub-stack separator plate 216 to the first pressure plate 204 to emergefrom the fourth port 510.

Analogous electrolyte flow paths may be implemented in the second cellstack 214 of the IFB 202, as shown in FIG. 6 . For example, positive ornegative electrolyte may flow into a fifth port 608 disposed in thesecond pressure plate 208, as indicated by arrows 610, travel upwards atvarious locations between the second pressure plate 208 and thesub-stack separator plate 216, as indicated by arrows 612, and exitthrough a sixth port 614, as indicated by arrows 616. The secondpressure plate 208 may also have a seventh port, aligned with the fifthport 608 along the x-axis, and an eighth port aligned with the sixthport 614 along the x-axis, also configured to flow negative or positiveelectrolyte through the second cell stack 214.

When flow of electrolyte between the first cell stack 212 and the secondcell stack 214 is inhibited by the sub-stack separator plate 216,independent operation of the cell stacks may be enabled. For example,the first cell stack 212 may be activated by turning on a firstelectrolyte pump, configured to drive electrolyte flow through the firstcell stack 212. Simultaneously, the second cell stack 214 may bedeactivated by maintaining electrolyte stagnant within the second cellstack 214. The electrolyte in the second cell stack 214 may be stagnantwhen a second electrolyte pump, configured to drive electrolyte flowthrough the second cell stack 214, is turned off or maintained off.Similarly, the first cell stack 212 may be deactivated by turning thefirst electrolyte pump off while the second cell stack 214 is activatedby turning the second electrolyte pump on. In this way, a variableamount of power may be derived from the double-stack IFB 202. Thedouble-stack IFB 202 may be fully deactivated, fully activated withelectrolyte flowing through both cell stacks, or configured to provide alower amount of power by operating the double-stack IFB 202 withelectrolyte flowing through one cell stack and not the other.

As another example, the double-stack IFB 202 may have ports in onepressure plate but not the other pressure plate. For example, the firstpressure plate 204 may have the first port 504, the second port 506, thethird port 508 and the fourth port 510 while the second pressure plate208 has no ports. As such, the sub-stack separator plate 216 may beadapted with openings to allow electrolyte entering the first cell stack212 of the double-stack IFB 202 to continue flowing through thesub-stack separator plate 216 and into the second cell stack 214.Electrolyte may circulate through the second cell stack 214, return tothe first cell stack 212 through the openings in the sub-stack separatorplate 216 and exit the first cell stack 212 through the first pressureplate 204. In such a configuration, the cell stacks may be activatedseparately, thus operation of the IFB 202 includes flowing electrolytethrough both the first cell stack 212 and the second cell stack 214simultaneously. While power supply may not be varied when the sub-stackseparator plate 216 is adapted to fluidly couple the second cell stack214 to the first cell stack 212, operating controls may be simplifiedcompared to independent operation of the first cell stack 212 and thesecond cell stack 214.

It will be appreciated that the IFB 202 shown in FIGS. 2-3 and 5-7 is anon-limiting example and other examples may include variations inelectrolyte flow paths without departing from the scope of the presentdisclosure. For example, the electrolyte flow paths may instead enterthe IFB at an upper region of the IFB and exit the IFB at a lower regionof the IFB, or the electrolyte flow paths may enter and exit at amid-region between the upper and lower regions.

As electrolyte-facilitated reactions occur at positive and negativeelectrodes of a double-stack IFB, electrons may be channeled to currentcollectors via electrically conducting lines, such metallic wires,extending between each terminal of each electrode, e.g., terminals 40and 42 of FIG. 1 , and the current conductors. Each positive electrodeof each cell in a cell stack may be electrically coupled to a positivecurrent collector and each negative electrode of each cell in the cellstack may be electrically coupled to a negative current collector. Thepositive and negative current collectors may transport electrons to anexternal circuit to energize, for example, an electrically powereddevice, and do not themselves participate in redox chemistry in the IFB.

The current collectors may be formed of a conductive material, such as ametal or a resin, and dimensions, e.g., length, width, thickness, etc.,of the current collectors may affect a current density transmitted tothe electrically powered device. The positive current collector may bearranged at an opposite end of the cell stack from the negative currentcollector. For example, as shown in a second side view 700 of thedouble-stack IFB 202 in FIG. 7 , the first current collector 306 of thefirst cell stack 312 (also shown in FIG. 3 ) may be a first positivecurrent collector 306 located between the first pressure plate 204 andthe first end plate 304. The second current collector 316 of the firstcell stack 212 may be a first negative current collector 316 arrangedbetween the second end plate 310 and the sub-stack separator plate 216.

The second cell stack 214 may have a third current collector 702positioned between the second pressure plate 208 and the third end plate330 of the double-stack IFB 202. The third current collector 702 may bea second positive current collector 702. A fourth current collector 704may be a second negative current collector 704, arranged between thesub-stack separator plate 216 and the fourth end plate 332 of the secondcell stack 214.

Each of the first cell stack 212 and the second cell stack 214 is thusconfigured to be individual electrical circuits, electrically insulatedfrom one another by the sub-stack separator plate 216. Each of thecurrent collectors may be readily coupled to an external circuit todirect current to an external device.

The sub-stack separator plate 216, as described above, may provideelectrical isolation of the cell stacks in the double-stack IFB 202.Electrolyte may or may not be communicated between the cell stacksthrough the sub-stack separator plate 216, depending on a configurationof electrolyte flow channels in the IFB 202. For example, the sub-stackseparator plate 216 may be a solid plate, blocking flow between the cellstacks, as shown in FIG. 6 . Alternatively, the sub-stack separatorplate 216 may include openings or ports to accommodate electrolyte flowchannels between the cell stacks.

Implementing at least one double-stack IFB in an IFB system may enablevariable power supply to one or more external devices, as describedabove. For example, operation of the double-stack IFB may be adjusted bya controller, such as the controller 88 of FIG. 1 , to either increaseor decrease an amount of power generated by the double-stack IFB basedupon a detected demand for power. For example, when a power demand islow, the controller 88 may deactivate a first cell stack of thedouble-stack IFB while operating a second cell stack of the double-stackIFB. Power is thereby supplied by the second cell stack only. However,when an electrically coupled external device is determined to have ahigh energy draw, the controller may activate both the first and thesecond cell stack to increase an amount of power delivered by thedouble-stack IFB.

In some examples, more than one double-stack IFB may be implemented inan IFB system. In such systems, the more than one double-stack IFB maybe configured to stack on top of one another in a secure manner tomaintain a compact footprint of the IFB system. An example of an IFBsystem 800 with more than one double-stack IFB is shown in FIG. 8 . TheIFB system 800 includes a first double-stack IFB 802 and a seconddouble-stack IFB 804, the second double-stack IFB 804 stacked on top ofthe first double-stack IFB 802.

Each of the first double-stack IFB 802 and the second double-stack IFB804 may be similarly configured to the double stack IFB 202 of FIGS. 2-3and 5-7 . The first double-stack IFB 802 may be oriented so that a firstset of pressure plates 806 and a first sub-stack separator plate 808 ofthe first double-stack IFB 802 are aligned with the y-axis. Similarly,the second double-stack IFB 804 may be oriented so that a second set ofpressure plates 810 and a second sub-stack separator plate 812 of thesecond double-stack IFB 804 are also aligned with the y-axis. The seconddouble-stack IFB 804 is arranged above the first double-stack IFB 802along the y-axis so that upper edges 814 of the first set of pressureplates 806 are in contact with bottom edges 816 of the second set ofpressure plates 810.

The sets of pressure plates may be adapted with nesting detents to alignthe stacked double-stack IFBs and maintain the alignment of the stackeddouble-stack IFBs. For example, as depicted in an expanded view 850 inFIG. 9 of dashed area 818, the upper edges 814 of the first set ofpressure plates 806 may include a first, male half 852 of a nestingdetent 854 that protrudes upwards, along the y-axis. The bottom edges816 of the second set of pressure plates 810 may include a second,female half 856 of the nesting detent 854, the second half 856 of thenesting detent 854 protruding downwards along the y-axis.

A diameter 858 of the first half 852 of the nesting detent 854 may beslightly smaller than a diameter 860 of the second half 856 of thenesting detent 854 to allow the first half 852 to slide readily into thesecond half 856. Thus, when the second double-stack IFB 804 is stackedon top of the first double-stack IFB 802, the first half 852 of thenesting detent 854 is nested within the second half 856 of the nestingdetent 854 and sideways sliding of the first and second double-stackIFBs 802, 804 relative to one another is inhibited. Engagement of thefirst half 852 with the second half 856 of the nesting detent 854 alsoaligns the double-stack IFBs along the y-axis.

In addition, the nesting detent 854 may have a through-hole 862extending along the y-axis through both the first half 852 and thesecond half 856 of the nesting detent 854. The through-hole 862 may be apin hole or a bolt hole that, when a pin or bolt is inserted through thethrough-hole 862, secures the first half 852 of the nesting detent 854to the second half 856 of the nesting detent 854, thereby securing thefirst double-stack IFB 802 and the second double-stack IFB 804 to oneanother.

Incorporating more than one double-stack IFB in an IFB system may widena range of power supplied by the IFB system. For example, in the IFBsystem 800 shown in FIG. 8 , when a demand for power is low, a singlecell stack of one of the double-stack IFB s may be operated while theother cell stacks are deactivated. As an example, a first cell stack ofthe first double-stack IFB 802 shown in FIG. 8 may be operated byactuating an electrolyte pump and circulating electrolyte through thefirst cell stack. A second cell stack of the first double-stack IFB 802may be deactivated by turning off an electrolyte pump or maintaining thepump off so that electrolyte is stagnant within the second cell stack.Electrolyte pumps coupled to a first and a second cell stack of thesecond double-stack IFB 804 be similarly deactivated. Thus energy isprovided exclusively by the first cell stack of the first double-stackIFB 802.

When power demand is high, all cell stacks of both double-stack IFBs maybe operated to meet the energy demand. Alternatively, when power demandis moderate, one of the double-stack IFBs may be actively generatingpower while the other is turned off. As a result, wasteful operation ofthe IFB system, e.g., providing more power than requested, is mitigatedand a lifetime of IFB components may be prolonged via efficient usage.

In this way, an energy storage capacity of an IFB system may be enhancedby increasing a number of IFBs in the IFB system without proportionallyincreasing a number of hardware components. A number of pressure platesand fastening assemblies, e.g., bolts, nuts, etc., may be reduced by atleast half, thereby decreasing an overall cost of the IFB system. As anexample, two individual cell stacks may be combined into a singledouble-stack IFB, held together by one set of pressure plates and onefastening assembly. Electrical isolation between the cell stacks may bemaintained by arranging a sub-stack separator plate between the cellstacks.

The technical effect of configuring an IFB in an IFB system as adouble-stack IFB is that an energy storage capacity of the IFB may beincreased while reducing manufacturing costs.

The disclosure also provides support for a redox flow battery system,comprising: a first redox flow battery, a second redox flow battery,stacked above and in contact with the first redox flow battery along avertical axis of the redox flow battery system, the second redox flowbattery coupled to the first redox flow battery via nesting detents,wherein operation of the first redox flow battery and the second redoxflow battery is adjustable according to a power demand. In a firstexample of the system, bottom edges of the second redox flow battery arein contact with upper edges of the first redox flow battery. In a secondexample of the system, optionally including the first example, a firstseparator plate of the first redox flow battery is aligned with a secondseparator plate of the second redox flow battery along the verticalaxis. In a third example of the system, optionally including one or bothof the first and second examples, the first redox flow battery includesa first cell stack and a second cell stack aligned along a horizontalaxis perpendicular to the vertical axis. In a fourth example of thesystem, optionally including one or more or each of the first throughthird examples, the second redox flow battery includes a third cellstack and a fourth cell stack aligned along a horizontal axisperpendicular to the vertical axis. In a fifth example of the system,optionally including one or more or each of the first through fourthexamples, the nesting detents each include a first half located in thefirst redox flow battery and a second half in the second redox flowbattery, and wherein the first half is configured to slide into thesecond half. In a sixth example of the system, optionally including oneor more or each of the first through fifth examples, when the first halfand the second half of the nesting detents are coupled, the second redoxflow battery is maintained stationary relative to the first redox flowbattery along a horizontal plane perpendicular to the vertical axis. Ina seventh example of the system, optionally including one or more oreach of the first through sixth examples, the redox flow battery systemis provided to deliver a first amount of power when one of the firstredox flow battery or the second redox flow battery is operating and asecond, higher amount of power when both the first redox flow batteryand the second redox flow battery are operating.

The disclosure also provides support for an all-iron redox flow batterysystem, comprising: a first redox flow battery including a first cellstack and a second cell stack aligned along a longitudinal axis of theall-iron redox flow battery system, a second redox flow battery arrangedabove the first redox flow battery along a vertical axis perpendicularto the longitudinal axis, the second redox flow battery including athird cell stack, aligned along the vertical axis with the first cellstack, and a fourth cell stack, aligned along the vertical axis with thesecond cell stack, with the third cell stack and the fourth cell stackaligned along the longitudinal axis, and detents formed of first halvesand respective second halves, the first halves arranged in the firstredox flow battery and the second halves arranged in the second redoxflow battery, wherein coupling of the first halves to the second halvesmaintains an alignment of the second redox flow battery relative to thefirst redox flow battery. In a first example of the system, the firsthalves of the detents protrude upwards along the vertical axis fromupper edges of a first set of pressure plates of the first redox flowbattery. In a second example of the system, optionally including thefirst example, the respective second halves of the detents protrudedownwards along the vertical axis from lower edges of a second set ofpressure plates of the second redox flow battery. In a third example ofthe system, optionally including one or both of the first and secondexamples, the first halves of the detents have a smaller diameter thanthe respective second halves of the detents, and wherein the firsthalves are coupled to the respective second halves by sliding the firsthalves into the respective second halves. In a fourth example of thesystem, optionally including one or more or each of the first throughthird examples, the all-iron redox flow battery system provides power tomeet a power demand by pumping electrolyte through one or more of thefirst, second, third, and fourth cell stacks. In a fifth example of thesystem, optionally including one or more or each of the first throughfourth examples, the first redox flow battery and the second redox flowbattery are operated independent of one another.

The disclosure also provides support for a redox flow battery system,comprising: a first redox flow battery maintained aligned and stationaryrelative to a second redox flow battery, the first and second redox flowbatteries stacked vertically along a vertical axis and with respectiveedges of a first set of pressure plates of the first redox flow batteryand a second set of pressure plates of the second redox flow battery incontact with one another, by one or more nesting detents included in thefirst and second sets of pressure plates. In a first example of thesystem, the first redox flow battery includes a first compressionassembly and the second redox flow battery includes a second compressionassembly, the first compression assembly and the second compressionassembly configured to exert pressure on the first set of pressureplates and the second set of pressure plates, respectively. In a secondexample of the system, optionally including the first example, the firstcompression assembly is configured to exert pressure on the first set ofpressure plates and the second compression assembly is configured toexert pressure on the second set of pressure plates along a longitudinalaxis of the redox flow battery system, the longitudinal axisperpendicular to the vertical axis and parallel with an alignment of afirst cell stack with a second cell stack of the first redox flowbattery and with an alignment of a third cell stack with a fourth cellstack of the second redox flow battery. In a third example of thesystem, optionally including one or both of the first and secondexamples, the first cell stack and the second cell stack are compressedbetween the first set of pressure plates by the first compressionassembly, and wherein the third cell stack and the fourth cell stack arecompressed between the second set of pressure plates by the secondcompression assembly. In a fourth example of the system, optionallyincluding one or more or each of the first through third examples, thefirst cell stack is separated from the second cell stack by a firstseparator plate and the third cell stack is separated from the fourthcell stack by a second separator plate, and wherein the first separatorplate and the second separator plate are electrically insulating. In afifth example of the system, optionally including one or more or each ofthe first through fourth examples, the redox flow battery system is aself-supporting unit that does not include an external housing.

The following claims particularly point out certain combinations andsub-combinations regarded as novel and non-obvious. These claims mayrefer to “an” element or “a first” element or the equivalent thereof.Such claims should be understood to include incorporation of one or moresuch elements, neither requiring nor excluding two or more suchelements. Other combinations and sub-combinations of the disclosedfeatures, functions, elements, and/or properties may be claimed throughamendment of the present claims or through presentation of new claims inthis or a related application. Such claims, whether broader, narrower,equal, or different in scope to the original claims, also are regardedas included within the subject matter of the present disclosure.

1. A redox flow battery system, comprising: a first redox flow battery;a second redox flow battery, stacked above and in contact with the firstredox flow battery along a vertical axis of the redox flow batterysystem, the second redox flow battery coupled to the first redox flowbattery via nesting detents, wherein operation of the first redox flowbattery and the second redox flow battery is adjustable according to apower demand.
 2. The redox flow battery system of claim 1, whereinbottom edges of the second redox flow battery are in contact with upperedges of the first redox flow battery.
 3. The redox flow battery systemof claim 1, wherein a first separator plate of the first redox flowbattery is aligned with a second separator plate of the second redoxflow battery along the vertical axis.
 4. The redox flow battery systemof claim 1, wherein the first redox flow battery includes a first cellstack and a second cell stack aligned along a horizontal axisperpendicular to the vertical axis.
 5. The redox flow battery system ofclaim 1, wherein the second redox flow battery includes a third cellstack and a fourth cell stack aligned along a horizontal axisperpendicular to the vertical axis.
 6. The redox flow battery system ofclaim 1, wherein the nesting detents each include a first half locatedin the first redox flow battery and a second half in the second redoxflow battery, and wherein the first half is configured to slide into thesecond half.
 7. The redox flow battery system of claim 6, wherein whenthe first half and the second half of the nesting detents are coupled,the second redox flow battery is maintained stationary relative to thefirst redox flow battery along a horizontal plane perpendicular to thevertical axis.
 8. The redox flow battery system of claim 1, wherein theredox flow battery system is provided to deliver a first amount of powerwhen one of the first redox flow battery or the second redox flowbattery is operating and a second, higher amount of power when both thefirst redox flow battery and the second redox flow battery areoperating.
 9. An all-iron redox flow battery system, comprising: a firstredox flow battery including a first cell stack and a second cell stackaligned along a longitudinal axis of the all-iron redox flow batterysystem; a second redox flow battery arranged above the first redox flowbattery along a vertical axis perpendicular to the longitudinal axis,the second redox flow battery including a third cell stack, alignedalong the vertical axis with the first cell stack, and a fourth cellstack, aligned along the vertical axis with the second cell stack, withthe third cell stack and the fourth cell stack aligned along thelongitudinal axis; and detents formed of first halves and respectivesecond halves, the first halves arranged in the first redox flow batteryand the second halves arranged in the second redox flow battery, whereincoupling of the first halves to the second halves maintains an alignmentof the second redox flow battery relative to the first redox flowbattery.
 10. The all-iron redox flow battery system of claim 9, whereinthe first halves of the detents protrude upwards along the vertical axisfrom upper edges of a first set of pressure plates of the first redoxflow battery.
 11. The all-iron redox flow battery system of claim 10,wherein the respective second halves of the detents protrude downwardsalong the vertical axis from lower edges of a second set of pressureplates of the second redox flow battery.
 12. The all-iron redox flowbattery system of claim 9, wherein the first halves of the detents havea smaller diameter than the respective second halves of the detents, andwherein the first halves are coupled to the respective second halves bysliding the first halves into the respective second halves.
 13. Theall-iron redox flow battery system of claim 9, wherein the all-ironredox flow battery system provides power to meet a power demand bypumping electrolyte through one or more of the first, second, third, andfourth cell stacks.
 14. The all-iron redox flow battery system of claim9, wherein the first redox flow battery and the second redox flowbattery are operated independent of one another.
 15. A redox flowbattery system, comprising: a first redox flow battery maintainedaligned and stationary relative to a second redox flow battery, thefirst and second redox flow batteries stacked vertically along avertical axis and with respective edges of a first set of pressureplates of the first redox flow battery and a second set of pressureplates of the second redox flow battery in contact with one another, byone or more nesting detents included in the first and second sets ofpressure plates.
 16. The redox flow battery system of claim 15, whereinthe first redox flow battery includes a first compression assembly andthe second redox flow battery includes a second compression assembly,the first compression assembly and the second compression assemblyconfigured to exert pressure on the first set of pressure plates and thesecond set of pressure plates, respectively.
 17. The redox flow batterysystem of claim 16, wherein the first compression assembly is configuredto exert pressure on the first set of pressure plates and the secondcompression assembly is configured to exert pressure on the second setof pressure plates along a longitudinal axis of the redox flow batterysystem, the longitudinal axis perpendicular to the vertical axis andparallel with an alignment of a first cell stack with a second cellstack of the first redox flow battery and with an alignment of a thirdcell stack with a fourth cell stack of the second redox flow battery.18. The redox flow battery system of claim 17, wherein the first cellstack and the second cell stack are compressed between the first set ofpressure plates by the first compression assembly, and wherein the thirdcell stack and the fourth cell stack are compressed between the secondset of pressure plates by the second compression assembly.
 19. The redoxflow battery system of claim 17, wherein the first cell stack isseparated from the second cell stack by a first separator plate and thethird cell stack is separated from the fourth cell stack by a secondseparator plate, and wherein the first separator plate and the secondseparator plate are electrically insulating.
 20. The redox flow batterysystem of claim 15, wherein the redox flow battery system is aself-supporting unit that does not include an external housing.